This invention relates to installing mechanical tube plugs and, in particular, to a method and apparatus for installing mechanical tube plugs including recording and monitoring the torque applied in the process to evaluate the work done and assure the integrity of the plug installation.
A heat exchanger in a nuclear steam supply system extracts heat from a closed primary loop fluid that circulates through the reactor and the heat exchanger and transfers the heat to a closed secondary loop fluid that circulates through the heat exchanger and a steam turbine where energy is extracted from the closed secondary loop fluid in the form of mechanical energy. The heat exchanger is comprised of a vessel containing a plurality of U-shaped tubes around which the fluid of the secondary loop passes and through which the fluid of the primary loop circulates. Heated water from the reactor enters the vessel in which the heat exchanger is housed beneath a tube sheet in the first side of a divided plenum. The heated water passes into and through the U-shaped tubes with both ends terminating in the tube sheet, thence into the second side of the divided plenum, thence from the vessel back to the reactor for reheating. While passing through the U-shaped tubes, the two fluids are heat exchange relationship with commingling of the fluids prevented only by the integrity of the heat exchanger tubes. It is imperative to maintain the fluids of the two loops isolated one from the other. Over the life of the nuclear steam supply system, some of the tubes of the heat exchanger deteriorate. During outages, the tubes of the heat exchanger are nondestructively tested to ascertain which tubes, if any, should be removed from service to assure the integrity of the heat exchanger. Since the heat exchanger is typically designed with more than the minimally necessary number of tubes, some tubes may be removed from service and the heat exchanger placed back in operation.
One method of removing a heat exchanger tube from service is to plug both ends of the tube where the tube terminates in the tube sheet. One method of plugging a tube end includes inserting a mechanical plug having an outside diameter slightly less than the inside diameter of the tube being plugged, the tube plug being inserted flush with the tube sheet. The plug is expanded by rolling the inner surface of the plug forming a mechanical rolled joint between the outer surface of the tube plug and the inside surface of the tube.
The rolling equipment, whether manual or automatic, is typically designed to stall at a predetermined torque. The integrity of the mechanical rolled joint is inferred from the torque at which the rolling equipment stalls. The rolling equipment is calibrated periodically to insure a consistent, acceptable rolled joint. However, as the rolling equipment is used repetitively, temperature changes in the clutch mechanism that stalls the rolling equipment at a predetermined torque can cause the rolling equipment to stall at different torque values thereby negating the integrity of the mechanical rolled joint. The effects of lubricant in the rolling equipment and the influence of temperature on the lubrication can have similar consequences thereby placing the integrity of a mechanically rolled joint, rolled by equipment that is designed to stall at a predetermined torque, in question.
A method and apparatus for installing mechanical tube plugs in which the actual torque applied in the installation process is monitored and recorded is needed to assure the integrity of the plug installation. Such a method and apparatus would permit real time monitoring of the torque applied in each step of the tube plug installation process, such that after each step in the mechanical tube plug installation process, the integrity of that step could be assured and further after the installation of a mechanical tube plug is complete, the integrity of the mechanical rolled joint may be assured and documented by the recording of the torque during each step of the installation process.